By screening and early detection of cancer, the Pap smear has decreased mortality from cervical cancer in the US by 70 percent over the last 40 years. Development of an analogous test for all solid tumors might have a similar impact on overall cancer mortality. Towards this end, a large literature has evolved describing methods of detection of circulating cancer cells. All methods described to date are dependent on affinity interactions (either epitopic or hybridization). The method described in this proposal uses a unique highly sensitive density-based method with the potential to become a simple office-based test that can detect circulating tumor cells in the peripheral blood. The test, called QBCytology is based on the QBCTM system which allows rapid, inexpensive determination of standard hematological blood tests including hematocrit, lymphocyte, granulocyte and platelet counts. This technique and the reader device is already in use in many doctors and veterinarians offices across the country. Although the QBC system depends only on low magnification images of the separations, in pilot studies with high power examination of the tubes we have shown that single cultured tumor cells can be identified microscopically by both morphology and epitopic markers. The sensitivity of the test is in the range of 1 cell in 106 to 107 nucleated cells, about equivalent to the sensitivity of PCR. Examination of blood from patients with known metastatic cancer shown morphologically atypical cells, some of which could be shown, epitopically, to be of epithelial lineage. A small blinded study was done on a cohort of 46 patients including 29 with known metastasis and 17 patients with no known cancer. Using morphologic techniques alone, we were able to identify 23 of the 29 cancer patients as a positive while all 17 negatives were correctly identified (sensitivity 79 percent, specificity, 100 percent). In summary, this density based assay is unique from all other current assays that attempt to detect circulating cancer cells. It has the potential to be an inexpensive, point-of-care, screening procedure for all epithelial neoplasms. This proposal describes pilot testing of the technique in larger populations and further studies to optimize the assay. If successful, we project future testing of this system as a method of cancer staging, and as a mechanism for evaluation of anti- cancer therapies. Given recent literature suggesting that tumor cells may circulate before metastasis, we also project future testing of this assay as screening test for early detection of solid tumors.